Researchers found female mice that were vaccinated before they got pregnant had babies with no sign of Zika infection.

"There are several vaccines in human trials right now, but to date, none of them has been shown to protect during pregnancy. We tested two different vaccines, and they both provided substantial protection," said co-senior study author Dr. Michael Diamond, a professor at Washington University School of Medicine in St. Louis.

Though Zika doesn't make most people seriously ill, it can be devastating for fetuses, causing problems with brain development, including an abnormally small head. Zika can also restrict babies' growth in the womb or trigger a miscarriage, researchers said.

The team tested the ability of two experimental vaccines to protect fetuses whose mothers were infected with Zika during pregnancy.

One vaccine -- based on the genetic blueprint for two proteins from the virus -- is already being tested in men and in women who are not pregnant. The other vaccine is a live but weakened form of Zika being tested in animals.

The team gave one of the two vaccines or a placebo to about 20 female mice. Some got a second dose. Both vaccines produced "very high levels" of anti-Zika antibodies.

Six days after the mice got pregnant, they were infected with Zika, to mimic the experience of a woman infected in early pregnancy.

Both vaccines effectively protected baby mice from the Zika virus. But the live-virus vaccine was most effective: just 22 percent of placentas and 17 percent of fetuses had traces of the virus, according to the study published July 13 in the journal Cell.

"The amount of viral genetic material in the placentas and fetuses from the vaccinated females was just above the limit of detection," said Diamond, a professor of molecular microbiology, pathology and immunology. "It's not totally clear whether it was infectious virus or just remnants of viruses that had already been killed."

Virus levels in unvaccinated mice were thousands of times higher, researchers said.

In experiments with different mice, researchers found unvaccinated mothers became very sick, their fetuses had high levels of infection and many died in the womb. The vaccinated females, however, stayed healthy and had babies with no signs of Zika infection, they added.

The authors said more research is needed to determine if the vaccines are safe and effective for pregnant women. The results of animal studies may not apply to humans. It's also unclear if these vaccines protect against the virus when it's transmitted through sex, instead of mosquito bites.

The study offers the first evidence that a vaccine given before pregnancy can protect fetuses from harmful effects of the mosquito-borne virus.

"In general, most doctors don't want to vaccinate during pregnancy on the outside chance that the immune response itself could harm the fetus," Diamond said.

"But if you're in an area where Zika is circulating, you might vaccinate during pregnancy because the risk of Zika infection is worse than some theoretical risk of immune-mediated damage," he added.